Frank,
If done properly, prison labor programs are the best way to go. The primary benefit is that it gets the inmates (my go to term since I am not up on all the current PC titles) up and working, learning skills, gives them a sense of accomplishment, holds them accountable in real world ways, and when managed properly it gives them assets they can use in a limited way for extras (earned reward) and it builds a nest egg for when they get out. For the state the benefit is that it occupies the inmates - "Idle hands are the devil's workshop" Proverbs, it generates a small inflow to help with expenses, and it improves the overall moral of the population.
As to companies purchasing from these programs: If a prison is participating in these programs, once their products enter the supply chain they are not segregated. Hogs, poultry, dairy, beef, etc. supplied by a prison farm goes into processing right along with items provided by thousands of other farms.
If there is a gripe concerning "slavery," that gripe goes to the state prison system, and to federal if you don't get satisfaction locally.
One of the worst things that happened in Minnesota was when Minnesota Correctional Facility - Stillwater shut down Minnesota Implement. This was a prison run farm implement manufacturing company. The products were like 99% of other ag equipment suppliers - Common, basic, simple, and highly plagiarized by every maker. MCF-Stillwater turned out excellent quality, no frills wagons, running gears, manure handling, feed handling, etc. all made on site by inmates, with inmates engineering, managing, and participating in marketing. Private business caused the shut-down, and immediately the competing products experienced very high inflation rates, and small ag supply companies lost access to their supply chain. Even worse, a few years later there became an issue with inmates being released after serving their time and no longer coming out with valuable trade skills they had learned. MCF-Stillwater went from being a correctional institution to a daycare center for criminals. They only education available was fine tuning your criminal skills as you were housed with people who's chosen profession and expertise was crime.
There is an excellent book written by Charles Colson and Jack Eckerd - Why America Doesn't Work (1992). The book is dated and needs a sequel, but an excellent view from a man who spent time for a crime and researched the good and bad in our systems.